Editing Proton (section)

{{Short description|Subatomic particle with positive charge}}
{{hatgrp|{{Other uses}}
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{{Lead too long|date=March 2025}}
{{Infobox particle
| bgcolour       =
| classification = [[Baryon]]
| name           = Proton
| image          = Quark structure proton.svg
| caption        = The [[valence quark]] content of a proton. The [[Color charge|color assignment]] of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by [[gluons]].
| num_types      =
| composition    = 2 [[up quark]]s (u), 1 [[down quark]] (d)
| statistics     = [[Fermionic]]
| group          = [[Hadron]]
| generation     =
| interaction    = [[Gravity]], [[Electromagnetic interaction|electromagnetic]], [[Weak interaction|weak]], [[Strong interaction|strong]]
| antiparticle   = [[Antiproton]]
| theorized      = [[William Prout]] (1815)
| discovered     = Observed as H<sup>+</sup> by [[Eugen Goldstein]] (1886). Identified in other nuclei (and named) by [[Ernest Rutherford]] (1917–1920).
| symbol         = {{SubatomicParticle|Proton}}, {{SubatomicParticle|Proton+}}, {{SubatomicParticle|Nucleon+}}, {{chem|1|1|H|+}}
| mass           = {{physconst|mp}}<br />{{physconst|mp_Da}}<br />{{physconst|mpc2_MeV|unit={{val|ul=MeV/c2}}}}
| mean_lifetime   = > {{val|3.6|e=29|u=years}}<ref>{{Cite journal|last1=The SNO+ Collaboration|last2=Anderson|first2=M.|last3=Andringa|first3=S.|last4=Arushanova|first4=E.|last5=Asahi|first5=S.|last6=Askins|first6=M.|last7=Auty|first7=D. J.|last8=Back|first8=A. R.|last9=Barnard|first9=Z.|last10=Barros|first10=N.|last11=Bartlett|first11=D.|date=2019-02-20|title=Search for invisible le modes of nucleon decay in water with the SNO+ detector|url=https://link.aps.org/doi/10.1103/PhysRevD.99.032008|journal=Physical Review D|volume=99|issue=3|pages=032008|doi=10.1103/PhysRevD.99.032008|arxiv=1812.05552 |bibcode=2019PhRvD..99c2008A |s2cid=96457175 }}</ref> (stable)
| electric_charge = {{val|p=+|1|ul=e}}
| charge_radius = {{val|0.8414|(19)|u=[[Femtometre|fm]]}}<ref name="CODATA2018" />
| electric_dipole_moment = < {{val|2.1|e=-25|u=''e''⋅cm}}<ref>{{Cite journal|last=Sahoo|first=B. K.|date=2017-01-17|title=Improved limits on the hadronic and semihadronic $CP$ violating parameters and role of a dark force carrier in the electric dipole moment of $^{199}\mathrm{Hg}$|url=https://link.aps.org/doi/10.1103/PhysRevD.95.013002|journal=Physical Review D|volume=95|issue=1|pages=013002|doi=10.1103/PhysRevD.95.013002|arxiv=1612.09371 |s2cid=119344894 }}</ref>
| electric_polarizability = {{val|0.00112|(4)|u=fm<sup>3</sup>}}
| magnetic_moment = {{physconst|mup}}<br />{{physconst|mup/muB|after=&nbsp;[[Bohr magneton|''μ''<sub>B</sub>]]}}<br />{{physconst|mup/muN|after=&nbsp;[[Nuclear magneton|''μ''<sub>N</sub>]]}}
| magnetic_polarizability = {{val|1.9|(5)|e=-4|u=fm<sup>3</sup>}}
| spin = {{sfrac|1|2}}&nbsp;[[reduced Planck constant|''ħ'']]
| isospin = {{sfrac|1|2}}
| parity = +1
| condensed_symmetries = ''[[Isospin|I]]''(''[[Total angular momentum|J]]''<sup>''[[Parity (physics)|P]]''</sup>) = {{sfrac|1|2}}({{sfrac|1|2}}<sup>+</sup>)
}}
A '''proton''' is a stable [[subatomic particle]], symbol {{SubatomicParticle|Proton}}, [[Hydron (chemistry)|H<sup>+</sup>]], or <sup>1</sup>H<sup>+</sup> with a positive [[electric charge]] of +1&nbsp;''e'' ([[elementary charge]]). Its mass is slightly less than the mass of a [[neutron]] and approximately {{val|1,836}} times the mass of an [[electron]] (the [[proton-to-electron mass ratio]]). Protons and neutrons, each with a mass of approximately one [[atomic mass unit]], are jointly referred to as ''[[nucleon]]s'' (particles present in atomic nuclei).

One or more protons are present in the [[Atomic nucleus|nucleus]] of every [[atom]]. They provide the attractive electrostatic central force which binds the atomic electrons. The number of protons in the nucleus is the defining property of an element, and is referred to as the [[atomic number]] (represented by the symbol ''Z''). Since each [[chemical element|element]] is identified by the number of protons in its nucleus, each element has its own atomic number, which determines the number of atomic electrons and consequently the chemical characteristics of the element.

The word ''proton'' is [[Greek language|Greek]] for "first", and the name was given to the hydrogen nucleus by [[Ernest Rutherford]] in 1920. In previous years, Rutherford had discovered that the [[hydrogen]] nucleus (known to be the lightest nucleus) could be extracted from the nuclei of [[nitrogen]] by atomic collisions.<ref name="Britannica" /> Protons were therefore a candidate to be a fundamental or [[elementary particle]], and hence a building block of nitrogen and all other heavier atomic nuclei.

Although protons were originally considered to be elementary particles, in the modern [[Standard Model]] of [[particle physics]], protons are known to be composite particles, containing three [[valence quark]]s, and together with [[neutron]]s are now classified as [[hadron]]s. Protons are composed of two [[up quark]]s of charge +{{sfrac|2|3}}''e'' each, and one [[down quark]] of charge −{{sfrac|1|3}}''e''. The [[rest mass]]es of quarks contribute only about 1% of a proton's mass.<ref name="Mass" /> The remainder of a proton's mass is due to [[quantum chromodynamics binding energy]], which includes the [[kinetic energy]] of the quarks and the energy of the [[gluon]] fields that bind the quarks together. The [[root mean square]] [[charge radius]] of a proton is about 0.84–0.87&nbsp;[[femtometre|fm]] ({{val|1|u=fm}} = {{val|e=-15|u=m}}).<ref name="PSI" /><ref name="Antognini2013" /> In 2019, two different studies, using different techniques, found this radius to be 0.833&nbsp;fm, with an uncertainty of ±0.010 fm.<ref name="ReferenceA">{{Cite journal|last1=Bezginov|first1=N.|last2=Valdez|first2=T.|last3=Horbatsch|first3=M.|last4=Marsman|first4=A.|last5=Vutha|first5=A. C.|last6=Hessels|first6=E. A.|s2cid=201845158|date=2019-09-06|title=A measurement of the atomic hydrogen Lamb shift and the proton charge radius|journal=Science|volume=365|issue=6457|pages=1007–1012|doi=10.1126/science.aau7807|pmid=31488684|issn=0036-8075|bibcode=2019Sci...365.1007B|doi-access=free}}</ref><ref name="ReferenceB">{{Cite journal|last1=Xiong|first1=W.|last2=Gasparian|first2=A.|last3=Gao|first3=H.|last4=Dutta|first4=D.|last5=Khandaker|first5=M.|last6=Liyanage|first6=N.|last7=Pasyuk|first7=E.|last8=Peng|first8=C.|last9=Bai|first9=X.|last10=Ye|first10=L.|last11=Gnanvo|first11=K.|s2cid=207831686|date=November 2019|title=A small proton charge radius from an electron–proton scattering experiment|journal=Nature|volume=575|issue=7781|pages=147–150|doi=10.1038/s41586-019-1721-2|pmid=31695211|bibcode=2019Natur.575..147X|osti=1575200|issn=1476-4687}}</ref>

Free protons occur occasionally on Earth: [[thunderstorm]]s can produce protons with energies of up to several tens of [[Electronvolt|MeV]].<ref name="Kohn2015" /><ref name="Kohn2017" /> At sufficiently low temperatures and kinetic energies, free protons will bind to [[electron]]s. However, the character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the [[electron cloud]] of an atom. The result is a diatomic or [[polyatomic ion]] containing hydrogen. In a vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral [[hydrogen atom]], which is chemically a [[free radical]]. Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules (H<sub>2</sub>), which are the most common molecular component of [[molecular clouds]] in [[interstellar medium|interstellar space]].<ref>{{Cite journal |last=Schlemmer |first=Stephan |date=2011-02-08 |title=H<sub>2</sub> Generation in the Early Universe Governs the Formation of the First Stars |url=http://dx.doi.org/10.1002/anie.201005920 |journal=Angewandte Chemie International Edition |volume=50 |issue=10 |pages=2214–2215 |doi=10.1002/anie.201005920 |pmid=21305679 |issn=1433-7851}}</ref>

Free protons are routinely used for accelerators for [[proton therapy]] or various particle physics experiments, with the most powerful example being the [[Large Hadron Collider]].